1. Field of the Invention
This invention relates to fluorescent lamps and the manufacture of fluorescent lamps, and, more particularly, to a method and apparatus for applying a layer of particulate material to the inside of a fluorescent lamp bulb by electrostatic deposition and the resulting fluorescent lamp.
2. Description of the Prior Art
In the prior art techniques for manufacturing fluorescent lamps, phosphor coatings are typically applied as a suspension of particulate material in a slurry including an organic binder. The organic binder serves the function of holding the phosphor particles to the glass bulb surface during the manufacturing of the bulb. After application of the phosphor coating, the bulbs are lehred at a high temperature to vaporize the organic binder and bond the phosphor particles to the glass bulb surface and to other phosphor particles to form a uniform, well-bonded coating on the fluorescent lamp bulb. This technique requires heating of the lamp bulb to a temperature which would cause the lamp glass to soften. To prevent distortion of the fluorescent lamp bulb, straight line fluorescent lamps are conventionally rotated during the lehring process so that the gravitational effects are averaged and the lamp maintains a straight shape.
U-shaped fluorescent lamps having both sets of lamp terminals at the same end of the lamp raise a difficulty with respect to lamp coating and lehring which is not experienced in manufacturing straight fluorescent lamps. In prior art techniques of manufacturing U-shaped fluorescent lamps, the phosphor coatings are typically applied as water suspensions containing organic polymer binders which act as dispersing agents to provide smooth coating appearance. After the coatings have been applied, the binders must be removed prior to sealing of the lamp and filling with the typical fluorescent lamp atmospheres, because the organic materials of the binder are incompatible with the fluorescent lamp atmosphere and tend to cause darkening and loss of lamp efficacy in lumens per watt over the life of the lamp. The binders typically are removed by baking at elevated temperatures, i.e. lehring, for a sufficient time to vaporize the binders. When folded fluorescent lamp tubes are subjected to lehring temperatures typically used for lehring lamps coated with water-based organic binder coatings (600.degree.-630.degree. C.), the glass can soften resulting in distortion of the glass tube due to gravity. It is impractical to roll the folded tube during the lehring process to average gravitational effects, and, therefore, lehring must occur at lower temperatures. However, lower temperature lehring significantly lowers lamp efficacy and maintenance due to the incomplete removal of the organic binder materials. In one prior art technique for manufacturing U-shaped fluorescent lamps a tin oxide starting strip is applied to an interior surface of the fluorescent lamp extending generally from one electrode around the bend of the lamp to the opposite electrode in order to assist in starting of the lamp. If this coating is applied prior to lamp bending, difficulties are experienced in maintaining electrical continuity of the starter strip following bending of the glass tube due to the strain on the glass and therefore on the starting strip during bending. Therefore, the starting strip is typically applied after the glass tube has been bent into the desired U-shape. A difficulty experienced when using tin oxide as the starting strip results from the use of an insulating barrier coating on the tin oxide coating to overcome the poor adherence of phosphors to tin oxide and the tendency of the tin oxide to darken with exposure to the atmosphere inside the fluorescent lamp. To improve adherence of phosphor materials to the tin oxide coating, certain types of borates, e.g. calcium borate, are included within the binder material. Removal of the binders from the lamp following deposition of the phosphors requires a still higher lehring temperature when additional borate additives are used, which increases the risk of sag in the U-shaped lamps. To overcome these limitations in the manufacturing of U-shaped fluorescent lamps, a technique of applying phosphor coatings and bonding the coatings to the lamp glass without requiring high temperature lehring is required.